Robert Turner

We have developed a workflow for numerical investigation of magnetic resonance imaging RF coil arrays. Use of fully parameterized 3-D and RF circuit models, and customized scripts for several time-consuming post-processing steps resulted in fast and continuous (24/7) generation of useful and important results. The high simulation speed allowed sensitivity analysis to be conducted for the most important dependencies - array diameter, human model position and tuning condition. For a dual-row array of 280 mm in diameter, as compared with a dual-row array of 250 or 230 mm in diameter, the safety excitation efficiency is higher and a lower margin of error is needed to maintain safety in operation. The impact of the tuning conditions and head position on the spatial-average 10-gram SAR significantly increases when the distance between the human model and the array decreases.

ABSTRACT We have numerically investigated the safety implications of interleaved RF excitation, as proposed by Avdievitch (2011), when used with multi-row near field arrays at 300 MHz. In most configurations, for both circular polarized excitation and static RF shimming, interleaved excitation resulted in multiplication of power deposited in the entire human tissue by a factor approximately equal to the number of interleaves, and a significant increase of specific absorption ratio averaged over 10 gram of tissue (SAR10g). However, for a specific configuration, in which interleaved excitation at each time step resulted in significant power reflected by the entire array, and RF excitation profiles that were well separated in space, the SAR10g remained practically the same as with simultaneous excitation.

Introduction Perfusion imaging using magnetically labeled water as an endogenous tracer is capable of measuring cerebral blood flow (1). Several studies at different main magnetic field strengths B0 were performed over the last decade (2-4). Perfusion measurements at 7 T are expected to be more sensitive because of the increased signal-to-noise ratio (SNR) at higher field strengths. For quantification of perfusion in continuous arterial spin labeling (CASL) experiments, however, an exact estimation of the inversion efficiency α is required. As CASL experiments are based on the adiabatic inversion of the flowing spins several simulation studies were performed focusing on different experimental and physiological parameters of the adiabatic fast passage, such as the B1 field amplitude, the strength of the applied gradient and the blood flow velocity (5-7). The presented work, however, centers on issues arising with higher field strengths. At such field strengths (e.g., 7 T) the specific absorption rate (SAR) limits the application of radio-frequency pulses so the B1 field should be as low as possible. Therefore, we investigated whether it is possible to achieve sufficient inversion efficiencies in CASL experiments at high B0 and low B1 field strengths. Method The determination of the inversion efficiency was based on a solution of the Bloch equations using the hard-pulse approximation (8,9). Spin relaxation was included in the simulation. The magnetization was calculated recursively while the actual values of the frequency offset (determined by the applied gradient) and the B1 field were inserted into the solution at every integration step. The simulation of the time dependent magnetization was started far below resonance and ended above resonance at a distance of 3 cm. The step size for the simulation was decreased until the results were stable. The simulation was performed in dependence on the gradient strength and the amplitude of the B1 field assuming a main magnetic field strength B0 of 7 T. For comparison, the inversion efficiencies were also calculated at B0 field strengths of 3 T and 1.5 T. As an estimate, T1 and T2 relaxation times of 2000 ms and 250 ms for arterial blood at 7 T, of 1700 ms and 275 ms at 3 T and of 1200 ms and 300 ms at 1.5 T were assumed. It should be noted that slightly different values for T1 and T2 do not affect the results of the simulations substantially. As the inversion efficiency is relatively insensitive to the blood flow velocity within the physiological range (5) and as the influence of the cardiac cycle can be neglected as long as the labeling pulse comprises at least one cardiac cycle (7), a constant blood flow velocity of 20 cm/s was assumed. For better comparison the inversion efficiencies were also plotted against the adiabaticity factor β which is described elsewhere (7). As for high field strengths the SAR limits the application of RF pulses a low B1 field is desirable. Therefore, gradient strengths producing maximum inversion efficiencies at low B1 field amplitudes were calculated. Results Tab. 1: Inversion efficiencies at a mean velocity of 20 cm/s. Inversion efficiencies α in dependence on the amplitude of the B1 field and on the gradient strength are summarized in Tab. 1. The dependence of α on the adiabaticity factor β is shown in Fig. 1. Whereas the absolute values of the inversion efficiencies vary slightly with field strength B0 a very similar relationship between α and β was found for the different field strengths. An optimum adiabaticity of 3...4 was obtained (Fig. 1). Fig. 2 depicts the dependence of α on the amplitude of applied gradient strength at 7 T. As dictated by the SAR limit low B1 field amplitudes of 1.2 and 0.6 μT were assumed for the simulation. Maximum inversion efficiencies of about 88% (B1 = 1.2 μT) and 85% (B1 = 0.6 μT) were found at gradient strengths of 0.8 and 0.25 mT/m, respectively. Discussion and Conclusion Inversion efficiencies of CASL experiments were calculated numerically under realistic experimental and physiological assumptions. It was shown that the main magnetic field strength and the resulting T1 and T2 relaxation times have an influence of the adiabatic inversion of flowing spins. Hence, for an exact estimation of the inversion efficiency in CASL experiments the field strength B0 has to be considered. However, for all B0 field strengths a B1 field amplitude and a gradient strength resulting in an adiabaticity factor β of about 3...4 can be considered as an optimum. Low B1 field amplitudes as desired at high main magnetic field strengths such as 7 T are still able to produce a sufficient efficiency of the adiabatic spin inversion. This is achievable if a corresponding low gradient strength is applied. However, it has to be mentioned that the simulations were performed assuming a perfectly homogeneous main magnetic field B0. A realistic, more inhomogeneous main magnetic field could markedly influence the shape and the…

In evaluation of RF transmit array coils, realistic estimation of losses were included in simulations with the aim of obtaining a better match between experimental results and numerical predictions. This required customized design of lossy circuit components, to overcome the limitation of the available built-in capabilities of current simulation tools. Some of the more time-consuming post-processing stages were relocated into Matlab, speeding post-processing by up to a factor of 100. The resulting numerical data can support the fabrication of dual row array with as many as 8 elements in each row, and elements overlapped in the Z direction.

A key aspect of optimal behavior is the ability to predict what will come next. To achieve this, we must have a fairly good idea of the probability of occurrence of possible outcomes. This is based both on prior knowledge about a particular or similar situation and on immediately relevant new information. One question that arises is: when considering converging prior probability and external evidence, is the most probable outcome selected or does the brain represent degrees of uncertainty, even highly improbable ones? Using functional magnetic resonance imaging, the current study explored these possibilities by contrasting words that differ in their probability of occurrence, namely, unbalanced ambiguous words and unambiguous words. Unbalanced ambiguous words have a strong frequency-based bias towards one meaning, while unambiguous words have only one meaning. The current results reveal larger activation in lateral prefrontal and insular cortices in response to dominant ambiguous compared to unambiguous words even when prior and contextual information biases one interpretation only. These results suggest a probability distribution, whereby all outcomes and their associated probabilities of occurrence-even if very low-are represented and maintained.

The brain is the most complicated organ in the body. Indeed, the great Harvard neurologist Gerald Fischbach described the human brain as "the most complex structure in the known Universe". How are we to make sense of it? And how are we to connect operations which we perceive as activities of our minds with empirical processes that are observable in the laboratory?

To identify a shielding material compatible with optical head-motion tracking for prospective motion correction and which minimizes radio frequency (RF) radiation losses at 7 T without sacrificing line-of-sight to an imaging target. We evaluated a polyamide mesh coated with silver. The thickness of the coating was approximated from the composition ratio provided by the material vendor and validated by an estimate derived from electrical conductivity and light transmission measurements. The performance of the shield is compared to a split-copper shield in the context of a four-channel transmit-only loop array. The mesh contains less than a skin-depth of silver coating (300 MHz) and attenuates light by 15 %. Elements of the array vary less in the presence of the mesh shield as compared to the split-copper shield indicating that the array behaves more symmetrically with the mesh shield. No degradation of transmit efficiency was observed for the mesh as compared to the split-copper shield. We present a shield compatible with future integration of camera-based motion-tracking systems. Based on transmit performance and eddy-current evaluations the mesh shield is appropriate for use at 7 T.

The conceptual foundations and ontology of cognitive neuroscience are rarely analysed in cross-cultural perspective, although they are manifestly the outcome of historical currents in specifically Western psychological science. How robust such concepts are, and how generalizable to other cultures, is thus quite problematic. Users of empirical techniques in imaging neuroscience are now actively exploring such topics as attention, volition, emotion and empathy, but with little awareness of how well or badly these concepts can be translated. This essay addresses issues of cultural bias and the potentially misleading use of extended metaphors in the typical deployment of mentalistic terminology, and suggests that there may be alternative conceptualizations, perhaps inspired by phenomenology, which would have less cultural baggage. Ultimately, the most scientifically useful ontology for interpreting and predicting human action may result from an integration of high quality ethnographic r...

The effect of coronary flow on the signal intensity of gradient recalled echo (GRE) MR images of the canine heart was evaluated at 4 T. Using a fully instrumented canine model, hyperfusion (local adenosine infusion) and ischemia (local occlusion) were evaluated while coronary blood flow and venous oxygen tension were monitored. These studies demonstrated an increase in GRE signal intensity with increases in coronary blood flow and venous oxygenation. A local occlusion resulted in a small decrease in signal intensity in the affected area of the heart. The dependence of the signal changes on TE and TR indicated that the effects of high flow were associated with changes in the apparent T2 and T1. Both relaxation effects could reflect changes in tissue blood volume associated with the conditions studied. In addition, the apparent T2 changes were consistent with alterations in total tissue deoxyhemoglobin concentration and the changes in apparent T1 were consistent with inflow effects.

Water diffusion in brain tissue can now be easily investigated using magnetic resonance (MR) techniques, providing unique insights into cellular level microstructure such as axonal orientation. The diffusive motion in white matter is known to be non-Gaussian, with increasing evidence for more than one water-containing tissue compartment. In this study, freshly excised porcine brain white matter was measured using a 125-MHz MR spectrometer (3T) equipped with gradient coils providing magnetic field gradients of up to 35,000mT/m. The sample temperature was varied between -14 and +19°C. The hypothesis tested was that white matter contains two slowly exchanging pools of water molecules with different diffusion properties. A Stejskal-Tanner diffusion sequence with very short gradient pulses and b-factors up to 18.8ms/μm(2) was used. The dependence on b-factor of the attenuation due to diffusion was robustly fitted by a biexponential function, with comparable volume fractions for each component. The diffusion coefficient of each component follows Arrhenius behavior, with significantly different activation energies. The measured volume fractions are consistent with the existence of three water-containing compartments, the first comprising relatively free cytoplasmic and extracellular water molecules, the second of water molecules in glial processes, and the third comprising water molecules closely associated with membranes, as for example, in the myelin sheaths and elsewhere. The activation energy of the slow diffusion pool suggests proton hopping at the surface of membranes by a Grotthuss mechanism, mediated by hydrating water molecules.

Structural magnetic resonance imaging can now resolve laminar features within the cerebral cortex in vivo. A variety of intracortical contrasts have been used to study the cortical myeloarchitecture with the purpose of mapping cortical areas in individual subjects. In this article, we first briefly review recent advances in MRI analysis of cortical microstructure to portray the potential and limitations of the current state-of-the-art. We then present an integrated framework for the analysis of intracortical structure, composed of novel image processing tools designed for high resolution cortical images. The main features of our framework are the segmentation of quantitative T1 maps to delineate the cortical boundaries (Bazin et al., 2014), and the use of an equivolume layering model to define an intracortical coordinate system that follows the anatomical layers of the cortex (Waehnert et al., 2014). We evaluate the framework with 150μm isotropic post mortem T2(∗)-weighted images and 0.5mm isotropic in vivo T1 maps, a quantitative index of myelin content. We study the laminar structure of the primary visual cortex (Brodmann area 17) in the post mortem and in vivo data, as well as the central sulcus region in vivo, in particular Brodmann areas 1, 3b and 4. We also investigate the impact of the layering models on the relationship between T1 and cortical curvature. Our experiments demonstrate that the equivolume intracortical surfaces and transcortical profiles best reflect the laminar structure of the cortex in areas of curvature in comparison to the state-of-the-art equidistant and Laplace implementations. This framework generates a subject specific intracortical coordinate system, the basis for subsequent architectonic analyses of the cortex. Any structural or functional contrast co-registered to the T1 maps, used to segment the cortex, can be sampled on the curved grid for analysis. This work represents an important step towards in vivo structural brain mapping of individual subjects.

We numerically investigated several magnetic resonance imaging radiofrequency transmit coil arrays, with and without a local shield, and with a range of scanner bore configurations. The latter had a significant influence on safety excitation efficiency. It is therefore important to include the scanner magnet room in the simulation domain, when the scanner bore is not isolated from the rest of the scan room by an electric shield or RF absorber. All arrays investigated provided similar inhomogeneity over the entire brain for a given excitation condition. However, for CP excitation mode transmit excitation efficiency was found to be higher for an array without a local shield.

ABSTRACT We evaluated the performance of purely cylindrical and conical-cylindrical non-overlapped dual-row near-field head arrays with radiative elements of axial length 70 and 80 mm. Increasing the filling factor using a conical row does not ensure an increase of the RF magnetic field averaged over the entire human brain. For the geometries, excitation conditions and volume of interest that were investigated, the conical shape resulted in an intrinsic reduction of safety excitation efficiency.

ABSTRACT We evaluated the performance of overlapped 8 and 16 channel dual-row near-field arrays with radiative elements of axial length 90 mm, after optimization of both array circuit and near-field behavior at 297.2 MHz. For each number of channels, an array performance measure was evaluated for four human head-and-torso model positions and three circuit level optimization strategies. For both channel numbers, this measure was very similar when optimizations were applied based either on “mode” or power reflected by the entire array. Reducing the number of array channels simplified array construction, at the cost of an intrinsic reduction of safety excitation efficiency and complexity of circuit level optimization.

It is now feasible to create spatial maps of activity in the human brain completely non-invasively using magnetic resonance imaging. Magnetic resonance imaging (MRI) images in which the spin magnetization is refocussed by gradient switching are sensitive to local changes in magnetic susceptibility, which can occur when the oxygenation state of blood changes. Cortical neural activity causes increases in blood

First of all, we would like to state that we are pleased that our paper has spawned a vivid debate about the validity of DCM. Given that DCM has been around for so many years, we think that this was long overdue. In the following, we would like to respond to the comments by Friston et al. and Breakspear.

The increased availability of ultra-high-field (UHF) MRI has led to its application in a wide range of neuroimaging studies, which are showing promise in transforming fundamental approaches to human neuroscience. This review presents recent work on structural and functional brain imaging, at 7 T and higher field strengths. After a short outline of the effects of high field strength on MR images, the rapidly expanding literature on UHF applications of blood-oxygenation-level-dependent-based functional MRI is reviewed. Structural imaging is then discussed, divided into sections on imaging weighted by relaxation time, including quantitative relaxation time mapping, phase imaging and quantitative susceptibility mapping, angiography, diffusion-weighted imaging, and finally magnetization-transfer imaging. The final section discusses studies using the high spatial resolution available at UHF to identify explicit links between structure and function. Copyright © 2015 John Wiley & Sons, Ltd.

Neuroanthropology is a new field of research that can make two distinctive contributions to our understanding of the brain-culture nexus. The first contribution has to do with the question of how socially shared meanings and practices are reflected in brain function and structure - the culture in the brain problem. Neuroanthropology's second contribution relates to the neural processes that generate socially shared meanings and practices - the brain in culture problem. Research in cultural neuroscience has focused on the first question while research in social neuroscience has a bearing on the second. A neuroanthropological perspective is vital to integrate these two most important dimensions of the human condition. In this paper we review research from cultural anthropology, primatology, and developmental psychology, in addition to social and cultural neuroscience, that deals with these two core neuroanthropological issues. Regarding the brain in culture problem, the review rev...

A conclusive mapping of myeloarchitecture (myelin patterns) onto the cortical sheet and, thus, a corresponding mapping to cytoarchitecture (cell configuration) does not exist today. In this paper we present a generative model which can predict, on the basis of known cytoarchitecture, myeloarchitecture in different primary and non-primary cortical areas, resulting in simulated in-vivo quantitative T1 maps. The predicted patterns can be used in brain parcellation. Our model is validated using a similarity distance metric which enables quantitative comparison of the results with empirical data measured using MRI. The work presented may provide new perspectives for this line of research, both in imaging and in modelling the relationship with myelo- and cytoarchitecture, thus leading the way towards in-vivo histology using MRI.

Changes in breathing change the concentration of oxygen and carbon dioxide in arterial blood resulting in changes in cerebral blood flow (CBF). This mechanism can be described by the cerebral vascular response (CVR), which has been shown to be altered in different physiological and pathophysiological states. CBF maps of grey matter (GM) were determined with a pulsed arterial spin labelling technique at 3 T in a group of 19 subjects under baseline conditions, hypoxia, and hypercapnia. Experimental conditions allowed a change in either arterial oxygen (hypoxia) or carbon dioxide (hypercapnia) concentration compared with the baseline, leaving the other variable constant, in order to separate the effects of these two variables. From these results, maps were calculated showing the regional distribution of the CVR to hypoxia and hypercapnia in GM. Maps of CVR to hypoxia showed very high intra-subject variations, with some GM regions exhibiting a positive response and others a negative response. Per 10% decrease in arterial oxygen saturation, there was a statistically significant 7.0 +/- 2.9% (mean +/- SEM) increase in GM-CBF for the group. However, 70% of subjects showed an overall positive CVR (positive responders), and the remaining 30% an overall negative CVR (negative responders). Maps of CVR to hypercapnia showed less intra-subject variation. Per 1 mm Hg increase in partial pressure of end-tidal carbon dioxide, there was a statistically significant 5.8 +/- 0.9% increase in GM-CBF, all subjects showing an overall positive CVR. As the brain is particularly vulnerable to hypoxia, a condition associated with cardiorespiratory diseases, CVR maps may help in the clinic to identify the areas most prone to damage because of a reduced CVR.

In this paper we present a multivariate analysis of evoked hemodynamic responses and their spatiotemporal dynamics as measured with fast fMRI. This analysis uses standard multivariate statistics (MANCOVA) and the general linear model to make inferences about effects of interest and canonical variates analysis (CVA) to describe the important features of these effects. We have used these techniques to characterize the form of hemodynamic transients that are evoked during a cognitive or sensorimotor task. In particular we do not assume that the neural or hemodynamic response reaches some "steady state" but acknowledge that these physiological changes could show profound task-dependent adaptation and time-dependent changes during the task. To address this issue we have modeled hemodynamic responses using appropriate temporal basis functions and estimated their exact form within the general linear model using MANCOVA. We do not propose that this analysis is a particularly powerful way to make inferences about functional specialization (or more generally functional anatomy) because it only provides statistical inferences about the distributed (whole brain) responses evoked by different conditions. However, its application to characterizing the temporal aspects of evoked hemodynamic responses reveals some compelling and somewhat unexpected perspectives on transient but stereotyped responses to changes in cognitive or sensorimotor processing. The most remarkable observation is that these responses can be biphasic and show profound differences in their form depending on the extant task or condition. Furthermore these differences can be seen in the absence of changes in mean signal.